A self-consistent approximation to the effective fermion mass of the standard
NJL model is expressed by the gap equation, which can be
obtained following Feynman-Dyson perturbation theory,
cf. [1]}, [2]}.
We established in [3]} that the gap equation of an extended NJL model
such as (REF ), which includes additional bilinear terms, has the same
general structure as that of the standard NJL model:
\(m = m_0 + \frac{2 i G N_c N_f}{(2\pi )^4} \, I,\) 
